utils: add formatter for rjson::value

before this change, we rely on the default-generated fmt::formatter
created from operator<<, but fmt v10 dropped the default-generated
formatter.

in this change, we define formatters for rjson::value, and drop its
operator<<().

Refs #13245

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#16956

utils/rjson.cc

@@ -565,7 +565,3 @@ sstring quote_json_string(const sstring& value) {
 }
 
 } // end namespace rjson
-
-std::ostream& std::operator<<(std::ostream& os, const rjson::value& v) {
-    return os << rjson::print(v);
-}

utils/rjson.hh

@@ -397,6 +397,9 @@ public:
 
 } // end namespace rjson
 
-namespace std {
-std::ostream& operator<<(std::ostream& os, const rjson::value& v);
-}
+template <> struct fmt::formatter<rjson::value> {
+    constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }
+    auto format(const rjson::value& v, fmt::format_context& ctx) const {
+        return fmt::format_to(ctx.out(), "{}", rjson::print(v));
+    }
+};